Current limiting fuse



April 13, 1948. c. L. scHuCK CURRENT LIHITING FUSE Filed oet. 1v, 1944 C w @www Si @Lw V I www Figs.

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Patented Apr. 13, 1948 CURRENT LMTIN G FUSE Hill, Pa., assignor to General Electric Company, a corporation of New Carl L. Schuck, Drexel York Application October 17, 1944, Serial No. 559,062

2 Claims.

My invention relates to fuses and more particularly to fuses of the current limiting type.

CurrentI limiting fuses have recently become very important and are now extensively used. Such current limiting fuses usually comprise one or more conductors of a suitable metal such as silver or zinc embedded in an inert, granular, refractory arc extinguishing material of high dielectric strength, such as sand, or relatively finely divided quartz. The whole construction is enclosed within'a housing having terminals for the fusible element mounted thereon.

Upon subjection of such fuse to currents of large magnitude such as upon short circuit, the fusible element attains fusing temperature and dissipates or vaporizes throughout a fusible section of considerable length as distinguished from dissipation at a point of reduced cross section, whereupon arcing occurs and the metal vapors rapidly expand to many times the Volume originally occupied by the fusible element itself.

The force of this expansion throws the metal vapors out of the arc gap and into the-spaces between the granules of the inert iiller material. These vapors condense upon the surfaces of the relatively cool granules and no longer are available for current conduction so that arcing is confined to the small space previously occupied by the fusible element. The intimate physical contact between the hot arc column and the relatively ool granules causes an exceedingly rapid transfer of heat from the arc to the granules so that the arc energy is dissipated in this heat transfer with practically negligible net pressure at the fuse housing.

As a result of the actions referred to in the preceding paragraph there is in effect suddenly inserted into the path of the current at the instant of melting of the fusible element a high resistance formed by the are column which initially limits the current to a magnitude which is only a fraction of the current which the system with which the current limiting fuse is associated, is The inserted resistance causes rapid current decay and subsequent interruption of the current flow with negligible mechanical disturbance, there being no gassing or discharge of vapors or particles and practically no noise.

It is an object of my invention to provide a new and improved current limiting fuse of the type described above.

It is another object of my invention to provide an arc extinguishing filler material for current limiting fuses having a grain size within definite limits so as to greatly increase the interrupting ability of the current limiting fuse.

Further objects and advantages of my invention will become apparent as the following description proceeds and the features of novelty which characterize my invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

For a better understanding of my invention reference may be had to the accompanying drawing in which Fig. 1 is a view partly in section of a current limiting fuse embodying my invention, and Figs. 2, 3, and 4 are curve diagrams to aid in understanding my invention.

Referring now to Fig. 1 of the drawing, I have disclosed a current limiting fuse comprising a tubular enclosing casing i which is constructed of glass, porcelain or other suitable insulating material Metallic terminal ferrules 2 and 3 are secured respectively on opposite ends of the casing I by means of any suitable form of glass to metal seal, indicated at 4. A fusible element supporting member 5 is provided at each end of the casing I and is preferably supported as by being welded or otherwise secured within ferrules 2 and 3, as indicated at 6. Interconnecting supporting members 5 are a plurality of fusible elements 'l connected in parallel with each other. These parallel fusible elements are constructed in accordance with the arrangement disclosed and claimed in Lohausen Patent 2,157,907, granted May 9, 1939, and'asslgned to the same assignee as the present application. In accordance with the above mentioned Lohausen patent each of the fusible elements has a section 'la of one cross sectional area, a section 'lb of another cross sectional area, and a section 'lc of still another cross sectional area. With this arrangement operation of the current limiting fuse results in the progressive insertion of resistance into the current path, thereby to reduce the magnitude of the surge voltages which are produced.

The upper end of the casing l is closed by means of an end cap 8. The lower end of the fuse casing is closed by a. relatively stiff partition member 9 and a flexible disk i 0, which latter is adapted to serve as an indicator or target and as illustrated is identical with the indicator disclosed and claimed in Rankin Patent 2,188,816, granted January 30, 1940, and assigned to the same assignee as the present application. The operation of the indicator comprising exible disk I0 is clearly described in the above mentoned Rankin patent and no further description i Y supporting 3 thereof will be included herein since it forms no part of the present invention.

It should be understood that the parallel fusible elements I which are illustrated as having no supporting means other than the members 5 may, if desired, be wound on a suitable core or means such as is illustrated in the above mentioned Rankin and Lohausen patents. By means of such a supporting member the eilective length of the fusible element may be considerably increased without increasing the length of the housing.

My invention is particularly concerned with the inert, granular arc extinguishing material II with which casing I is filled and in which the fusible elements 1 connected in parallel with each other are completely embedded.

I have discovered that a greatly improved interrupting ability of a current limiting fuse may be obtained by a proper choice of the grain size of the inert arc extinguishing material I I. I have discovered that for a particular diameter of wire forming the fusible elements of the current limiting fuse the interrupting ability varies greatly with the grain size. I have made extensive tests of interrupting ability versus grain size of the inert arc extinguishing material Il in current limting fuses of the type shown in Fig. 1, for

wire sizes in the range between and 30 mils diameter.

The results of these tests where the fusible element comprises silver wire having a diameter in one case of 14 mils, in another case of 18 mils, and in still another case of mils, are clearly illustrated in Fig. 2. In making these particular tests, the circuit impedance was held constant and the voltage was varied to bracket the successful and unsuccessful interruption. The abscissa in Fig. 2 are expressed in the number of openings per square inch in the finest mesh screens through which the granular material would pass. In this connection the coarsest mesh screen was used first, followed by successively finer mesh screens. In Fig. 3 there is illustrated acurve which relates the grain size in terms of screen mesh to the diameter in inches of the aperture according to the British Institute of Mining and Metallurgy standard scale. From Fig. 2 it is obvious that the optimum grain size for silver wire in the range between 10 and mils diameter is in the neighborhood of 120 mesh, or .00425 inch with reference to Fig. 3. I have found that a granular quartz sand iiller having a grain size so that it will pass through a 115 mesh screen but not through a 170 mesh screen provides a very desirable filler and increases the interrupting capacity by a considerable amount. For a 115 mesh screen the openings in the U. S. standard Tyler screens are .0049 inch or .125 millimeter, while for the 1'70 mesh screen the opening size is .0035 inch or .088 millimeter.

The procedure for extracting the desired grain sizes from an inert arc extinguishing material which usually contains grain sizes varying between less than 50 to over 250 mesh, is accomplished in the following manner: A certain amount of inert granular siliceous material, such as sand, is placed on a, 115 mesh standard mesh' screen and the screen is then shaken. The sand which passes through the 115 mesh screen is then placed on a 170 mesh screen which i-s also shaken. The sand which idoes not pass through the 170 mesh screen is the desired product for filling the casing of current limiting fuses such as is illustrated in Fig. 1, whereby the greatly increased interrupting capacity is obtained. It Will b 4 understood that in the final product there will still be a small percentageof inert material finer than 170 mesh which just did not happen to be exposed to the screen long enough to nd ts way through.

It is believed that interrupting ability increases as the grain size of the material decreases up to an optimum value around 120 mesh and thereafter decreases is as follows: When the inert lling material becomes too fine it presents practically a solid wall and acts as an impediment to the dissipation of silver vapor through it upon dissipation of the silver fuse wires. On the other hand when the granular inert filler material il is too coarse the silver f vapor spreads through it so rapidly that the arc becomes dangerously large in diameter which is also an impediment to satisfactory interruption. In addition I have found that the presence of iron or its oxides in sand is detrimental to interrupting ability. It has been discovered by others and checked in the ease of the sands used by me as a fuse filler that the iron oxide content of sand increases as the grain size becomes smaller, that is as the mesh number increases. In Fig. 4 there is illustrated a curve setting for the percentage of iron oxide as a function of the grain size of the sand used for these data. This sand was chosen from a number of commercially available sand products for its relative freedom from iron contamination. It is observed that the percentage of iron oxide increases very rapidly for grain sizes beyond 170 mesh. Thus the decreased interrupting ability as the grain sizes decrease beyond 170 mesh, as is indicated in Fig. 2, may further be explained by the increased iron oxide contamination. By excluding grain sizes smaller than 170 mesh much of the iron oxide contamination is eliminated and an analysis of the inert, granular arc extinguishing material having a grain size between and 170 mesh was found to have an ,iron oxide content in the neighborhood of .04 -per cent.

It may be observed from Fig. 2 that there is a slight variation in the optimum grain size for the different wire diameters. Accordingly, in certain cases it might be desirable where several diameter wires are employed as in the fuse in Fig. 1, to iill the lower portion of the fuse casing I containing the fusible section 1a which may be, for example, 25 mil wire, with the optimum grain size material as determined from Fig. 2, then filling the portion of casing I surrounding fusible section 'Ib which may have. a diameter of 14 mils for example, with the optimum grain size as determined from Fig. 2, and then filling the remainder of the casing I within which the fusible sections 'Ic are contained, which may have a diameter of 18 mils with the optimum grain size filling material as determined from Fig. 2. With this arrangement the maximum overall interrupting ability is obtained. In view of the fact that the optimum grain size is very similar for wire diameters between l0 and 30 mils, the casing I is preferably lled with a uniform grain size material fallingwithin the range between 115 and mesh.

By means of the use of an inert arc extinguishing material of a grain size falling within the range set forth above, I have found it possible to increase the interrupting ability of a 60G-volt. 10-ampere current limiting fuse from 50,000 to 100,000 amperes.

Heretofore the grain sizes employed in current limiting fuses have been for the most part coarser the explanation of why the the iron oxide contamination may become serious.

While I have shown than 100 mesh and and described a particular embodiment of my invention, it will be obvious to those skilled in the art that various changes and modiiications may be made without departing from my invention in its broader aspects and I therefore aim in the appended claims to cover all such modifications and changes as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure 'by Letters Patent of the United States is:

1. A current limiting fuse comprising, in combination, a casing, a fusible element mounted in v said casing having a diameter between about 10 and 30 mils and an inert granular arc quenching siliceous material within said casing embedding said fusible element, substantially all of said 6 material including a mixture of all the grain sizes that will pass through a 115 mesh screen and be held on a 170 mesh screen.

2. A current limiting fuse comprising, in combination. a casing, a fusible element mounted in said casing and having a cross-sectional area between about 78 square mils and 707 square mils and sand within said casing embedding said fusible element, substantially4 all of said sand including all the natural grain sizes that will pass through a 115 mesh screen and be held on a 170 mesh screen.

CARL L SCHUCK.

REFEBEN CES CITED The following references are of record in the ille oi' this patent:

UNITED STATES PATENTS Number Name Date 2,135,166 Bussmann Nov. 1, 1938 r2,143,037 Smith Jan. 10, 1939 2,157,907 Lohausen May 9, 1939 2,180,975 Brown Nov. 21, 1939 2,223,959 .Lohausen Dec. 3, 1940 2,382,271 Suits et al. Aug. 14, 1945 2,408,351 Suits Sept. 24, 1946 

